Convert string to integer

/*Copyright Tom Lukeywood 2014
    This file is part of Convert string to interger.

    Convert string to interger is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    Convert string to interger is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with Convert string to interger edition.  If not, see <http://www.gnu.org/licenses/>.
*/

unsigned int atoi(char* s, unsigned char len);

unsigned int atoi(char* s, unsigned char len){
unsigned int val=0;
unsigned int flag=0;
unsigned char i;
for(i=0;i<len;i++)
{
    if(s[i] >= '0' && s[i] <= '9'){val = val * 10 + s[i] -'0';}
    else if(s[0] == '-' && i==0){flag = 1;}
    else{break;}
}
if(flag == 1)
val = val * -1;
return val;
}

example of the function being used:

char c[10] = "123";
int i = atoi(c, 3);
printf("%d", i);